1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to a liquid crystal display device and a method of fabricating a liquid crystal display device.
2. Discussion of the Related Art
In general, thin film transistor (TFT) liquid crystal display (LCD) devices are commonly used because of their superior resolution and thin profile. The TFT-LCD device includes a lower substrate having a TFT array substrate, an upper substrate having a color filter array substrate, a liquid crystal layer between the lower and upper substrates, and a backlight device provided below the lower substrate for emitting light through the device. Commonly, an amount of the light emitted from the backlight device and transmitted through the TFT and color filter array substrates is about 7%. Accordingly, it is necessary to improve luminous efficiency of the backlight device in order to obtain a high luminance LCD device. However, improving the luminous efficiency requires an increase in power consumption. For example, in portable LCD devices, such as a notebook computer, since large-capacity batteries are commonly used to provide the power required to drive the backlight device, overall weight of the portable LCD devices is relatively larger and time of use of the LCD devices is relatively short. In order to overcome these problems, a reflective-type LCD device, which does not require the use of the light emitted from the backlight device, has been developed.
The reflective-type LCD devices operate using ambient light, thereby significantly decreasing power consumption of the LCD devices. Accordingly, the reflective-type LCD devices are useful for portable display devices, such as electronic notebook or personal digital assistant (PDA) devices. The reflective-type LCD devices have opaque reflective plates or opaque reflective electrodes that correspond to pixel regions of transparent electrodes. However, the reflective-type LCD devices have low luminance since they use the ambient light. For example, in the reflective-type LCD devices, the ambient light is transmitted through the color filter substrate, and then the transmitted light is reflected by the reflective electrode of the lower substrate. Subsequently, the reflected light is transmitted back through the color filter substrate, thereby displaying an image. Accordingly, since the ambient light is transmitted through the color filter substrate twice, the light transmittance is lowered, thereby lowering luminance.
Generally, a thickness of the color filter is inversely proportional to light transmittance, and is proportional to color purity. Thus, decreasing the thickness of the color filter improves the light transmittance, but lowers the color purity. Since the color filter is formed of a resin, it is difficult to manufacture the color filter of a predetermined thickness. To overcome these problems, an LCD device that uses a cholesteric liquid crystal (CLC) layer is being developed that selectively reflects or transmits the ambient light.
Since a state of liquid crystal molecules is dependent upon their structure and composition, a liquid state is affected by temperature and density. For example, since nematic liquid crystal molecules having a regular arrangement along a predetermined direction, they are commonly used in LCD devices. Since, liquid crystal molecules of a CLC layer having chiral characteristics, wherein axes are twisted, a mixture with nematic liquid crystal molecules would result in directors of the nematic liquid crystal molecules becoming twisted. In general, the nematic liquid crystal molecules are regularly aligned along one direction, whereas the CLC layer has layered-structures such that the liquid crystal molecules of each layer have general nematic regularity. However, the liquid crystal molecules rotate about one direction between each of the layers, so that a reflexbility difference between the layers is generated. According to. the reflexibility difference, it is possible to display colors by light reflection and interference.
The liquid crystal molecules of the CLC layer are rotated in a helical structure, wherein the helical structure has two structural characteristics, such as rotation direction of helix and repetition period of helix, i.e., pitch. The pitch is a distance between points having the same alignment structure of the liquid crystal layer and is a variable for determining the color of the CLC layer. For example, a peak wavelength is a function of a product of the pitch and the average refractive index, λ=n(avg)·pitch, wherein n(avg) is the average refractive index of the liquid crystal. For example, when the pitch of the CLC having a average refractive index of 1.5 is 430 nm, the peak refractive wavelength is about 650 nm, thereby displaying a red color. Thus, by changing the pitch of the CLC, it is possible to obtain green and blue colors.
The rotation direction of the CLC helix is a characteristic of the CLC structure, which is an important factor in polarization of CLC reflectivity. That is, the circular polarizing direction of light is determined in accordance with the left or right polarization of the helical structure of the CLC. Accordingly, since the ambient light may be the mixture of left-circular polarized light and right-circular polarized light, it is possible to divide the left- or right-circular polarized light from the ambient light using the CLC. Thus, the CLC has greater light efficiency than that of the LCD device that makes use of linearly polarized light characteristics. Although the same power consumption is used, the CLC obtains great light transmittance as compared with that of the LCD device having color filters made of pigment or dye.
FIG. 1A and FIG. 1B are schematic cross sectional views of a reflective-type LCD device according to the related art. In FIGS. 1A and 1B, an upper transparent electrode 3 is formed on an upper transparent substrate 1, and receives a voltage from an external power source 17, wherein the upper transparent electrode 3 is formed of a transparent conductive metal, such as an Indium-Tin-Oxide ITO. Then, a first alignment layer 5 is formed on the upper transparent electrode 3 for an alignment of a liquid crystal layer, and a light-absorbing layer 11 is formed on a lower transparent substrate 9 for absorbing incident light. In addition, a lower transparent electrode 13 is formed on the light-absorbing layer 11 for applying a voltage to the liquid crystal layer, and a second alignment layer 15 is formed on the lower transparent electrode 13 for an alignment of the liquid crystal layer. Next, a CLC layer 7 is formed between the upper and lower transparent substrates 1 and 9 facing each other.
The CLC layer 7 selectively reflects circular polarized light having a predetermined wavelength to produce specific colors according to their wavelength. In addition, the CLC layer 7 functions as a reflective plate, thereby making it possible to omit an additional color filter and a reflective plate.
The CLC layer 7 may be used to control an ON/OFF mode of the power source 17. First, during an OFF mode of the power source 17, as shown in FIG. 1A, the liquid crystal molecules of the CLC layer 7 have a helical pitch (not shown), wherein a helical axis is formed along a vertical direction. Accordingly, when light is incident on the CLC layer 7, left-circular polarized light 23 having the same wavelength as the helical pitch (twisted extent of the liquid crystal layer) is completely reflected, and right-circular polarized light 21 is dispersed and transmitted through the CLC layer 7 to be absorbed by the light-absorbing layer 11. Meanwhile, if the liquid crystal molecules are twisted along an opposite direction, the right-circular polarized light 21 is completely reflected, and the left-circular polarized light 23 is completely absorbed.
Accordingly, a user observes the left-circular polarized light 23 or the right-circular polarized light 21 reflected by the CLC layer 7 as a white state. In addition, the light having the predetermined wavelength (i.e., the same wavelength as that of the helical pitch of the CLC) is reflected, so that the user observes light having a specific color according to the predetermined wavelength.
During the ON mode of the power source 17, as shown in FIG. 1B, the liquid crystal molecules of the CLC layer 7 are aligned along an electric field oriented in-plane to the upper and lower substrates 1 and 9, wherein the electric field is induced along the vertical direction between the upper and lower transparent electrodes 3 and 13. Accordingly, the helical axis of the CLC layer 7 is not formed so that the most of the left-circular polarized incident light 19a and the right-circular polarized incident light 19b is dispersed and transmitted through the CLC layer 7 to be absorbed by the light-absorbing layer 11. Thus, the user does not observe the reflected light, (i.e., a dark state).
When the external voltage is not applied from the power source 17, it is possible to selectively reflect the left-circular polarized light 23 (in FIG. 1A) or the right-circular polarized light 21 having the same wavelength as that of the helical pitch to the direction of the helical axis for being in-plane to the upper and lower substrates 1 and 9. Upon application of the external voltage from the power source 17, the helical axis disappears due to the vertical electric field, whereby the incident light 19 is absorbed by the light-absorbing layer 11, thereby displaying a black image (i.e., black state). Thus, a single color is generated by reflecting the light having the predetermined wavelength with a single CLC layer 7.
FIG. 2 is a cross sectional view of another LCD device according to the related art. In FIG. 2, an LCD device includes a first LCD panel 1, a second LCD panel 2, and a third LCD panel 3. The first LCD panel 1 forms a first CLC layer 7a having a helical pitch the same as a wavelength of light corresponding to a red color, the second LCD panel 2 forms a second CLC layer 7b having a helical pitch the same as a wavelength of light corresponding to a green color, and the third LCD panel 3 forms a third CLC layer 7chaving a helical pitch the same as a wavelength of light corresponding to a blue color.
When the light 19 is incident, the first, second, and third CLC layers 7a, 7b and 7c are twisted along a left-handed screw direction to reflect left-circular polarized light of a red color wavelength 25, a green color wavelength 27, and a blue color wavelength 29, respectively, thereby producing light having various colors. In addition, right-circular polarized light is dispersed and transmitted through the first, second, and third CLC layers 7a, 7b and 7c, and is absorbed by a light-absorbing layer 11 formed on a lower transparent substrate 9c. Accordingly, the first, second, and third CLC layers 7a, 7b and 7c are formed for reflecting the light corresponding to the red, green, and blue color wavelengths to produce light having various colors. However, the LCD device requires an increased number of substrates, thereby lowering contrast ratio and efficiency.